This invention relates to solid state electronic relays and more particularly to such relays utilizing photovoltaic isolators and field effect power transistors which exhibit break-before-make operation.
Solid state relays are well known for use in electrical power systems to control the energization of a load by a power source. Copending application Ser. No. 303,811, filed Sept. 21, 1981 and assigned to the present assignee discloses a solid state relay utilizing photovoltaic isolators and field effect power transistors and sets forth background material related to such relays which is herein incorporated by reference. The present invention differs from the disclosure of the copending application in that it includes provisions for break-before-make operation, means for limiting turn-off time of the field effect transistors, and changes in control logic circuitry.
Photovoltaic isolators provide a voltage output signal which can be used as the gate driving signal for a field effect transistor (FET) that serves as the switching element in a solid state relay. Due to the very small power output of these photovoltaic isolators, control of timing in the FET circuit is difficult. Typically, turn-off time can be several times longer than turn-on time. In some applications, it is desirable to produce a relay with break-before-make operation, that is, with a longer turn-on time than turn-off time.
A relay constructed in accordance with the present invention is provided with a control logic circuit which includes means for turning off at least one output pole prior to turning on at least one other pole. The control logic circuit receives a low level input control signal and controls at least two power field effect transistor switching elements by means of at least two photovoltaic optical isolators.
A plurality of operational modes are possible in a single circuit, including operation as a normally open relay, a normally closed relay, or a latched relay. A high degree of isolation is provided between the input control and power circuits by the optical isolator, while the power field effect transistor switch element provides for a low switch voltage drop.
The control logic circuit includes a voltage regulator, control input circuit, signal conditioning circuit, latching circuit and optical isolator driving circuit. Application of a positive control signal or a grounding signal to designated terminals in the control input circuit causes a change in the logic level of an exclusive OR gate output from a logic high to a logic low. This logic signal passes through a conditioning network which prevents noise from actuating the relay and provides a clean signal to the latching circuit. The latching circuit includes a flip-flop to provide the latching feature and two additional exclusive OR gates which amplify and invert the logic level control signal. The relay power supply can be connected to the flip-flop reset terminal to disable the latching feature.
The control signal passes from the latching circuit to an optical coupler driving circuit which includes transistor switches for driving light emitting diodes (LED's) in the optical coupler circuit and a logic circuit which controls the current to the LED's to ensure break-before-make operation. Each LED is contained in an optical coupler that includes an array of photovoltaic diodes which generates voltage in response to radiation from the LED's. This generated voltage is impressed on the gate of a power field effect transistor, thereby turning it on to provide the relay contact closure function. If the latching feature was not disabled, the FET will remain in the on condition even after the control input signal is removed.
The relay contact opening function is provided in a similar manner, except that the LED is turned off, thereby turning off the power FET. A voltage limiter limits maximum gate voltage to the FET's, thereby reducing turn-off time. Thus a relay in accordance with this invention exhibits a low switch voltage drop, provides for break-before-make operation and is capable of normally open, normally closed, or latched operation.